Hydraulic hoes are generally designed with a normal hoist capacity that is less than the possible maximum and is selected to balance such factors as efficiency and machine life. In some cases, for example where the operator desires to use the machine to lift a section of pipe or other load or to lift an overhanging slab, it is desirable to be able to hoist at or near actual maximum capacity. It is important, however, to prevent the machine from being used continuously in such a maximum hoist or lift mode, to avoid defeating the design objectives.
One way of increasing hoist capacity is to provide a dual pressure relief valve in the hoist circuit. A biasing signal is applied when the machine is to be used in maximum lift mode, thus effectively increasing the relief setting to allow increased hoist pressure and capacity. Preventing continuous use in this mode can be accomplished by simultaneously disabling the wrist function so the machine cannot be used for normal digging. A system incorporating this general approach is shown in U.S. Pat. No. 4,218,837, issued Aug. 26, 1980, but that system is not believed fully satisfactory. For one thing, it utilizes a second dual pressure relief valve in the wrist circuit to disable the wrist function, and the system can apparently be used only when the dipper is in a selected position.